Crystallization Mechanism of Regular- and Reverse-Graded Quasi-2D Perovskites

Quasi-2D perovskites have recently attracted great interest because of their structural tunability and enhanced stability compared to 3D perovskites. Quasi-2D films display a higher degree of complexity because multiple structural phases are formed during film crystallization. As a result, it is common to observe multidimensional films with both 2D and 3D phases forming a phase distribution gradient, where lower-dimensional phases locate at the bottom, and 3D at the top of the film. Recently, a few studies have found that it is possible to create a reverse-graded quasi-2D perovskite films, where 2D phases locate at the top and 3D ones at the bottom. Even though the crystallization mechanism of quasi-2D perovskites has been investigated before, no study has analyzed the crystallization differences between regular- and reverse-graded quasi-2D perovskites.<br/> <br/>In our work, we have fabricated quasi-2D perovskite films using spacers with increasing alky chain length, going from butylammonium to dodecylammonium. First, we found that alkyl spacer length plays a major role in the phase distribution gradient. Short spacers form a conventional 2D-3D gradient, with 2D phases at the bottom, whereas long spacers reverse the gradient. Interested by this behavior, we analyzed the crystallization mechanism via in-situ absorption measurements and described how film formation changes with increasing alkyl spacer length. With the help of these findings, we designed a new fluorinated spacer and used a multispacer approach to tune quasi-2D perovskite film properties, displaying yet another facet of their degree of tunability.